1. Field of the Invention
The present invention relates to a method of manufacturing a silver halide emulsion having high sensitivity and producing low fog and also relates to an emulsion and a silver halide light-sensitive material with high sensitivity and good graininess.
2. Description of the Related Art
Basic properties required for a photographic silver halide emulsion are high sensitivity, low fog, and fine graininess.
In order to increase the sensitivity of an emulsion the following is required, (1) increase the number of photons absorbed by a single grain; (2) increase the efficiency of converting photoelectrons generated by light absorption into a silver cluster (latent image); and (3) increase development activity for effectively utilizing the obtained latent image. Increasing the size increases the number of photons absorbed by a single grain but degrades image quality. Increasing the development activity is an effective means of increasing the sensitivity. In the case of parallel development as color development, however, the graininess is generally degraded. In order to increase the sensitivity without graininess degradation, it is most preferable to increase the efficiency of converting photoelectrons into a latent image, i.e., increase a quantum sensitivity. In order to increase the quantum sensitivity, low-efficiency processes such as recombination and latent image dispersion must be minimized. It is known that a reduction sensitization method of forming a small silver nucleus without development activity inside or on the surface of a silver halide is effective to prevent recombination.
James et al. have found that the sensitivity can be increased with a lower fog level than that in normal reduction sensitization when a kind of reduction sensitization, in which a coating film of an emulsion subjected to gold-plus-sulfur sensitization is vacuum-deaerated and then heat-treated in a hydrogen atmosphere, is performed. This sensitization method is well known as hydrogen sensitization and is effective as a lab-scale high sensitization means. The hydrogen sensitization is actually used in the field of astrograph.
Methods of reduction sensitization have been studied for a long time. Carroll, Lowe et al., and Fallens et al. disclose that a tin compound, a polyamine compound, and a thiourea dioxide-based compound are effective as a reduction sensitizer in U.S. Pat. Nos. 2,487,850 and 2,512,925 and British Patent 789,823, respectively. Collier compares properties of silver nuclei formed by various reduction sensitization methods in "Photographic Science and engineering", Vol. 23, P. 113 (1979) She used methods of dimethylamineborane, stannous chloride, hydrazine, high-pH ripening, and low-pAg ripening. Reduction sensitization methods are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867. Not only selection of a reduction sensitizer but also a method of using a reducing agent are disclosed in, e.g., JP-B-57-33572 ("JP-B-" means examined Japanese patent application), JP-B-58-1410, and JP-A-57-179835 ("JP-A-" means unexamined published Japanese patent application) Techniques of improving storage stability of an emulsion subjected to reduction sensitization are disclosed in JP-A-57-82831 and JP-A-60-178445. Regardless of a number of studies as described above, an increase in sensitivity is insufficient as compared with that obtained in hydrogen sensitization in which a light-sensitive material is treated with hydrogen gas in a vacuum. This is reported by Moisar et al. in "Journal of Imaging Science", Vol. 29. P. 233 (1985).
The conventional techniques of reduction sensitization are insufficient to satisfy a recent demand for a photographic light-sensitive material with high sensitivity and high image quality. The hydrogen sensitizing means also has a drawback in which a sensitizing effect is lost when a light-sensitive material is left in air after hydrogen sensitization. Therefore, it is difficult to utilize this sensitization method to prepare a photographic light-sensitive material for which no special apparatus can be used.